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SubjectRe: [PATCH v2 4/4] btrfs: replace btrfs_io_context::raid_map[] with a fixed u64 value
 	Hi Qu,

On Tue, 7 Feb 2023, Qu Wenruo wrote:
> In btrfs_io_context structure, we have a pointer raid_map, which is to
> indicate the logical bytenr for each stripe.
>
> But considering we always call sort_parity_stripes(), the result
> raid_map[] is always sorted, thus raid_map[0] is always the logical
> bytenr of the full stripe.
>
> So why we waste the space and time (for sorting) for raid_map[]?
>
> This patch will replace btrfs_io_context::raid_map with a single u64
> number, full_stripe_start, by:
>
> - Replace btrfs_io_context::raid_map with full_stripe_start
>
> - Replace call sites using raid_map[0] to use full_stripe_start
>
> - Replace call sites using raid_map[i] to compare with nr_data_stripes.
>
> The benefits are:
>
> - Less memory wasted on raid_map
> It's sizeof(u64) * num_stripes vs size(u64).
> It's always a save for at least one u64, and the benefit grows larger
> with num_stripes.
>
> - No more weird alloc_btrfs_io_context() behavior
> As there is only one fixed size + one variable length array.
>
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Thanks for your patch, which is now commit 4a8c6e8a6dc8ae4c ("btrfs:
replace btrfs_io_context::raid_map with a fixed u64 value") in
next-20230220.

noreply@ellerman.id.au reported several build failures when
building for 32-bit platforms:

ERROR: modpost: "__umoddi3" [fs/btrfs/btrfs.ko] undefined!

> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -6556,35 +6532,44 @@ int __btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
> }
> bioc->map_type = map->type;
>
> - for (i = 0; i < num_stripes; i++) {
> - set_io_stripe(&bioc->stripes[i], map, stripe_index, stripe_offset,
> - stripe_nr);
> - stripe_index++;
> - }
> -
> - /* Build raid_map */
> + /*
> + * For RAID56 full map, we need to make sure the stripes[] follows
> + * the rule that data stripes are all ordered, then followed with P
> + * and Q (if we have).
> + *
> + * It's still mostly the same as other profiles, just with extra
> + * rotataion.
> + */
> if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK && need_raid_map &&
> (need_full_stripe(op) || mirror_num > 1)) {
> - u64 tmp;
> - unsigned rot;
> -
> - /* Work out the disk rotation on this stripe-set */
> - rot = stripe_nr % num_stripes;
> -
> - /* Fill in the logical address of each stripe */
> - tmp = stripe_nr * data_stripes;
> - for (i = 0; i < data_stripes; i++)
> - bioc->raid_map[(i + rot) % num_stripes] =
> - em->start + ((tmp + i) << BTRFS_STRIPE_LEN_SHIFT);
> -
> - bioc->raid_map[(i + rot) % map->num_stripes] = RAID5_P_STRIPE;
> - if (map->type & BTRFS_BLOCK_GROUP_RAID6)
> - bioc->raid_map[(i + rot + 1) % num_stripes] =
> - RAID6_Q_STRIPE;
> -
> - sort_parity_stripes(bioc, num_stripes);
> + /*
> + * For RAID56 @stripe_nr is already the number of full stripes
> + * before us, which is also the rotation value (needs to modulo
> + * with num_stripes).
> + *
> + * In this case, we just add @stripe_nr with @i, then do the
> + * modulo, to reduce one modulo call.
> + */
> + bioc->full_stripe_logical = em->start +
> + ((stripe_nr * data_stripes) << BTRFS_STRIPE_LEN_SHIFT);
> + for (i = 0; i < num_stripes; i++) {
> + set_io_stripe(&bioc->stripes[i], map,
> + (i + stripe_nr) % num_stripes,

As stripe_nr is u64, this is a 64-by-32 modulo operation, which
should be implemented using a helper from include/linux/math64.h
instead.

> + stripe_offset, stripe_nr);
> + }
> + } else {
> + /*
> + * For all other non-RAID56 profiles, just copy the target
> + * stripe into the bioc.
> + */
> + for (i = 0; i < num_stripes; i++) {
> + set_io_stripe(&bioc->stripes[i], map, stripe_index,
> + stripe_offset, stripe_nr);
> + stripe_index++;
> + }
> }
>
> +
> if (need_full_stripe(op))
> max_errors = btrfs_chunk_max_errors(map);

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds

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